Minimally invasive surgery (MIS) provides surgical techniques for operating on a patient through small incisions using a camera and elongate surgical instruments introduced to an internal surgical site, often through trocar sleeves or cannulas. The surgical site often comprises a body cavity, such as the patient's abdomen. The body cavity may optionally be distended using a clear fluid such as an insufflation gas. In traditional minimally invasive surgery, the surgeon manipulates the tissues using end effectors of the elongate surgical instruments by actuating the instrument's handles while viewing the surgical site on a video monitor.
A common form of minimally invasive surgery is endoscopy. Laparoscopy is a type of endoscopy for performing minimally invasive inspection and surgery inside the abdominal cavity. In standard laparoscopic surgery, a patient's abdomen is insufflated with gas, and cannula sleeves are passed through small (generally ½ inch or less) incisions to provide entry ports for laparoscopic surgical instruments. The laparoscopic surgical instruments generally include a laparoscope (for viewing the surgical field) and instruments for performing surgical functions such as gripping or cutting.
Laparoscopic surgical instruments are similar to those used in conventional (open) surgery, except that the working end or end effector of each instrument is separated from its handle by a shaft. As used herein, the term “end effector” means the actual working part of the surgical instrument and can include clamps, graspers, scissors, staplers, image capture lenses, and needle holders, for example. To perform surgical procedures, the surgeon passes these surgical instruments through the cannula sleeves to an internal surgical site and manipulates them from outside the abdomen. The surgeon monitors the procedure by means of a monitor that displays an image of the surgical site taken from the laparoscope. Similar endoscopic techniques are employed in other types of surgeries such as arthroscopy, retroperitoneoscopy, pelviscopy, nephroscopy, cystoscopy, cisternoscopy, sinoscopy, hysteroscopy, urethroscopy, and the like.
Robotic control may provide an improved control interface to the surgeon. Robotically controlled surgical instruments may be driven by servo mechanisms, such as servo motors, that are coupled to the surgical instrument by mechanical cables. Each servo mechanism may be coupled to a cable by a driving pulley that draws in and pays out the cable wound around the driving pulley. The cable may be routed to and from the driving pulley by one or more guide pulleys. As space in the surgical field over a patient where robotically controlled surgical instruments are being used is at a premium, it is desirable to have a compact mechanism to drive and control the robotically controlled surgical instruments.
The cable may rotate a driven pulley that is coupled to the robotically controlled surgical instrument to drive and control movement of the instrument. A portion of the cable may be wound around the driving and/or driven pulley more than once to improve the transfer of force and motion between the cable and the pulley. The cable may be made up of two or more cable segments. The ends of segments may be attached to pulleys and these segment ends may be wound around the driving and/or driven pulley more than once to provide a larger range of motion. The driving and driven pulleys may be referred to as capstans in such configurations.
The cable may move a linearly driven mechanism, such as a carriage, rather than rotate a driven pulley. While the description will primarily discuss systems with a rotationally driven pulley, it should be understood that in many, but not all of the systems, a linearly driven mechanism may be substituted for the rotationally driven pulley.
The cable may be split at the driving and/or driven pulley with each of the cable segment ends coupled to the pulley to provide a positive connection between the cable and the pulley. It will be appreciated that the two cable segments will still continue to function as they would if the cable was wound around the pulley as a single continuous cable. References to a cable should be understood to include arrangements of two or more cable segments where the segments are operatively coupled, such as by being joined to a pulley that transfers force and motion from one segment to the other.
In a typical cable drive system for a robotically controlled surgical instrument, a cable is guided by a pulley and wound onto a pulley that is fixed to a shaft that is driven by a motor. The cable is normally adjusted to place the cable in tension. Maintaining tension in the cable may be necessary to keep the cable properly positioned on the guiding pulleys and the driving and driven pulleys.
The cable may stretch under an applied load. The stretch may be constructional and/or elastic. Constructional stretch is the result of the clearances between the individual wires and strands being reduced as the cable is loaded, allowing the cable to “stretch” in length. Elastic stretch is the actual elongation of the individual wires in a strand or cable when subjected to a load that is less than the yield point of the metal. When the load is removed from the cable it will return to its original length.
A cable can only transmit force or motion in tension. Thus the applied force must draw in the cable and pull on the driven end. In a system that uses a cable loop between the driving pulley and the driven pulley, one portion of the cable will be drawn in by the driving pulley. The tension in the drawn in portion of the cable may be increased substantially if the driven end is working against mechanical resistance. The increase in tension may cause this portion of the cable to stretch.
The other portion of the cable in a cable loop is payed out by the driving pulley and taken up by the driven pulley. The tension in the payed out portion of the cable may be reduced substantially, or even eliminated, if the driven end is working against mechanical resistance. Further, if the drawn in portion of the cable stretches, the increased length of the cable will also contribute to slack in the payed out portion of the cable.
It may be necessary to provide a mechanism to maintain tension in the payed out portion of the cable to keep the cable loop properly positioned. This may be accomplished by including an extension spring as part of the cable. However this requires that there be a portion of the cable with a distance between pulleys that is greater than the range of motion of the cable. A distance between the pulleys is necessary because the extension spring is normally substantially larger in diameter than the cable and thus cannot pass over the pulleys. Including an extension spring as part of the cable may be detrimental if the cable loop is required to provide positive force and/or positioning in both directions. Including an in-line spring as part of the cable loop may greatly increase the elasticity of the cable loop and adversely affect the ability to positively transmit force and/or motion between the driving pulley and the driven pulley.
It would be desirable to provide a mechanism that can compensate for stretching of a cable loop with positive transmission of force and/or motion without requiring additional space between pulleys.